Conventional radial packing rings for rotary motion have a packing lip of substantially truncated-cone configuration and composed of a structurally strong rubber material or a synthetic elastomer. The contact area between the packing lip and the rotary machine part to be sealed off or packed is very short so that its sharp edge presses against the machine part with the radial pressure inherent in the packing lip and/or that of a tension spring washer surrounding the packing lip in a circular groove thereof.
The packing effect results from the mutual pressure exerted by the rubber or elastomer surface and the surface of the machine part. There is only a small flexible engagement between the surfaces, and because of the roughness of the surfaces there is a minute clearance therebetween. A lubricant can leak through this clearance because of capillary forces, thereby limiting friction between the surfaces. Where one or both of the surfaces have a high degree of roughness, this clearance may be excessive so that the lubricant will then not be retained between the surfaces but may leak past the packing seal or bearing in prohibitive amounts. The interfacial forces operating between the surfaces and the surface tension of the lubricant must then be sufficiently strong to prevent such leakage.
The friction between the surfaces is a combination of solid friction, boundary-layer friction and liquid friction, all of which vary depending upon the lubrication conditions, the rotary speed and the degree to which the surfaces have been worn away by mutual contact during the rotary motion. Thus, the overall friction can vary substantially. Prior-art radial shaft-packing rings are characterized by a high overall friction, a narrow-contact surface and prohibitive leakage where mutual pressure is raised to limit leakage friction and wear increases.
It has proved to be difficult to lower the friction coefficient with conventional packing rings. The recommended maximum peripheral velocity of a standard ring may be, say 10 m/s. If this speed is exceeded, frictional heat may damage the packing ring. However, by making the packing ring of high-quality special rubber it has proved possible to increase the peripheral velocity to 25-30 m/s without causing material damage to the packing ring although the rotary machine part can be worn and ridged. A hardened surface of the rotary machine part cannot completely prevent such abrasion and wear and the sharp edge of the packing lip gradually cuts a groove in the rotary machine part surface. This causes clearance problems between the packing lip and the rotary machine part, requiring machining of the latter upon packing replacement.
Hydrodynamic packings use a gap which can be made fluid tight by means of formations on one or both of the relatively rotating members which induce a feed back of fluid to generate a hydrodynamic field which seals off the lubricant. Thus, the lubricant acts as its own barrier to leakage.
However, this sealing effect is only possible with some systems when the direction of rotation is opposite to the direction of the pitch of the back-feed formation and the packing must have an elongated cylindrical contact surface of approximately 5 mm and a helical formation of about two turns. Because the back-feed effect occurs only in one direction of rotation the use of such packing rings is naturally restricted. To overcome, this problem, packing rings have been developed of alternating twist so that the hydrodynamic action can be achieved in both directions of rotation. Capillary grease traps are arranged along the front edge of the conical packing surface. They are formed as flat isosceles triangles. Their acute angles point in the direction of the periphery and their legs are formed as sharp helical edges. The triangular areas can protrude from the packing surface with their apices adjacent the front edge of the packing ring. There can also be a small gap between the front edge of the packing ring and the circular line of triangular areas, the bases of the triangles rather than their apices now being adjacent the front edge. The effect of these two arrangements is that the helical edges always have the same pitch in relation to the lubricant body contacting the front edge of the packing ring, regardless of the direction of rotation of the shaft, and a back-feed effect results in either rotational sense. However, despite the presence of a hydrodynamic packing effect due to a back-feed helix configuration, the packing ring and also the shaft are subjected to considerable solid friction in addition to liquid and boundary friction, although the proportion of liquid friction now is substantially higher. Abrasion and wear can be noticeable. Further, the back-feed helix arrangement becomes inefficient as the abrasion of the contact surface of the packing ring continues, and the service life is severely limited.